1. Field of the Invention
The present invention relates to a zoom lens system, which is preferably used as an image taking optical system for a silver film camera, a digital still camera, a video camera and the like.
2. Related Background Art
Along with the enhancement in resolution of a digital still camera, i.e., an increase of the number of pixels in the digital still camera, there is a demand for correcting a monochromatic aberration and sufficiently correcting a chromatic aberration in a zoom lens as an image taking optical system for a solid-state image pickup element with a high resolution. In particular, when the focal length at a telephoto end of a zoom lens increases as a result of an increase in a zoom ratio or an imaging magnification, there is a demand for the reduction in a secondary spectrum, as well as the primary achromatism, regarding a chromatic aberration.
Conventionally, in order to correct a secondary spectrum of an axial chromatic aberration (longitudinal chromatic aberration) at a telephoto end, a number of zoom lenses using anomalous dispersion (extraordinary dispersion) glass have been known. Furthermore, as a zoom lens configuration suitable for a high zoom ratio, there is a positive lead type in which a lens unit closest to an object side has a positive refractive power.
For example, JP H06-43363 A and JP H03-58490 B (counterpart: U.S. Pat. No. 4,709,997) disclose an example using glass having anomalous dispersibility for a zoom lens in which three lens units having positive, negative, and positive refractive powers are arranged in this order from the object side.
For example, JP 3097399 B, JP 2002-62748 A (counterpart: U.S. Pat. No. 6,594,087), JP H08-248317 A, and JP 2001-194590 A (counterpart: U.S. Pat. No. 6,404,561) disclose an example using glass having anomalous dispersibility for a zoom lens in which four lens units having positive, negative, positive, and positive refractive powers are arranged in this order from the object side.
For example, JP 2001-350093 A (counterpart: U.S. Pat. No. 6,449,433) discloses an example using glass having anomalous dispersibility for a zoom lens in which five lens units having positive, negative, positive, negative, and positive refractive powers are arranged in this order from the object side.
Any of the above-mentioned conventional examples use glass having an Abbe number exceeding 80 for a positive lens in a first lens unit having a positive refractive power. In general, low dispersion glass having an Abbe number exceeding 80 has anomalous dispersibility. The use of such glass for a positive lens in a first lens unit of a positive lead type is effective for reducing a secondary spectrum at a telephoto end.
However, the above-mentioned documents disclose no example using a material having anomalous dispersibility for a negative lens in the first lens unit, which are focused on only the reduction in a secondary spectrum by use of the anomalous dispersion characteristics of a positive lens. In order to further reduce a secondary spectrum amount with such a configuration, it is considered to use a material having high anomalous dispersibility such as fluorite or to increase the number of positive lenses in the first lens unit. The former has a problem in terms of cost, and the latter has a problem in terms of enlargement.